Non-invasive radio frequency liquid level and volume detection system using phase shift

ABSTRACT

A medical fluid system includes a medical fluid pump; a container holding a fluid to be pumped by the medical fluid pump, the fluid at a first time having a first conductivity, the fluid at a second time having a second conductivity; and a radio frequency level sensor positioned in operable relation with the container, the radio frequency operation of the level sensor configured so as to be (i) indicative of a level of the fluid in the container and (ii) at least substantially independent of whether the fluid has the first conductivity or the second conductivity. The medical fluid system can determine the level of medical fluid by measuring the resistance, impedance or phase shift seen by the sensor.

This application claims priority to U.S. Patent Application Ser. No.61/450,452, entitled “NON-INVASIVE RADIO FREQUENCY LIQUID LEVEL ANDVOLUME DETECTION SYSTEM,” filed Mar. 8, 2011, and U.S. PatentApplication Ser. No. 61/451,725, entitled “NON-INVASIVE RADIO FREQUENCYLIQUID LEVEL AND VOLUME DETECTION SYSTEM USING PHASE SHIFT,” filed Mar.11, 2011.

BACKGROUND

The present disclosure relates to sensing the level of a medical fluidin a container.

Knowing a volume or level of a medical fluid in a container is importantin many medical fluid applications. For example, it may be important toknow when a medical fluid supply is running low, so that a new source offluid can be installed or opened. In a reusable container, it may beimportant to know the liquid level to ensure that the container is notoverfilled but has enough supply for whatever use is necessary.

Certain existing medical device sensors require that an associated pumpor moving part be stopped before a fluid level can be sensed accurately.Stopping the therapy to take a measurement results in a point in timesystem as opposed to a true real time system. Stopping therapy alsolengthens overall therapy time.

Other existing sensors use a capacitive probe or capacitive element thatmeasures a distance between the probe and the fluid. Capacitive sensorsrely on the conductivity of the fluid and thus may not be desirable inmedical applications in which the conductivity of a measured fluidchanges during therapy. For example, in dialysis applications, theconductivity of dialysis fluid may fluctuate as the fluid is regularlymodified, refreshed, and rejuvenated. Dialysis and other medical fluidapplications may accordingly provide a container that holds differentfluids having different conductivities at different times. In suchapplications, it may be advantageous to have a level sensor that is atleast substantially independent of or unaffected by fluid conductivity.

Another class of existing sensors relies on radiating a signal from atransmitter to a receiver and measuring the attenuation of the radiatedsignal to determine whether fluid exists at various points inside acontainer. Radiation in medical applications is undesirable because itcan interfere with nearby equipment and may be harmful to the patient.

Other existing sensors rely on invasive probes that must be in contactwith the measured fluid or need to be located inside the container wherethe measured fluid resides. The probes present sterility anddisinfection issues. Residue can build up on the probe, requiringadditional maintenance and cleaning. Probe systems can also makeswapping or changing fluid containers cumbersome. Opening a newcontainer to insert a probe requires the container to be openable andpresents further sterilizing issues. It would therefore also beadvantageous to have a non-invasive sensor that does not requirephysical contact with the fluid and does not need to be placed insidethe container.

A need accordingly exists for an improved medical tank level sensor.

SUMMARY

The present system and method involve a medical fluid application inwhich a level or volume of a medical fluid in a container is sensed andknown. The medical fluid system in one embodiment is a renal failuretherapy system, such as a hemodialysis (“HD”), hemofiltration (“HF”),hemodiafiltration (“HDF”) or continuous renal replacement (“CRRT”)system. The dialysis system is alternatively a peritoneal dialysis(“PD”) system. The dialysis system may be an online system, in whichdialysis fluid or dialysate is made during treatment. The dialysissystem may alternatively be a batch system, in which the dialysate ismade and stored for one or more treatment. The dialysis system mayfurther alternatively be a bagged system in which the dialysate ispremade before therapy.

In any of the above renal failure or dialysis systems, it iscontemplated that there is a container, e.g., rigid tank or bag, inwhich it is desirable to know the level or volume, and in which thecontainer at different times holds fluids having differingconductivities. For example, the online dialysis system can have theability to pump purified water or mixed dialysate. The water can bepumped to prime, flush and/or disinfect the dialysis system. The mixeddialysis fluid or dialysate is pumped during therapy. A fluid holdingtank is provided to hold a ready supply of either purified water ordialysate, so that if needed, an additional volume of fluid (water ordialysate) can be delivered to an associated dialyzer or hemofilter orelsewhere as needed. The level or volume detection system and method ofthe present disclosure is coupled operably to the tank as discussed indetail below to know, in real time, the level or volume of purifiedwater or dialysate in the holding tank. The measurement is accurateregardless of fluid conductivity, for example, regardless of whetherwater, dialysate, or some intermediate version thereof, is present inthe container.

In another dialysis example, a batch supply of dialysate is made at thesite of the dialysis therapy. For example, a batch supply of twenty toone-hundred liters is made in a batch container for one or more therapy.The batch container can be a container having a pre-supplied amount ofconcentrate to make a predefined volume of dialysate. Purified water isthen added to make the specified amount of dialysate. As water is added,the conductivity of the mixture changes—that is, lessens—as thepre-supplied concentrate is diluted to the desired level. The level orvolume detection system and method of the present disclosure is coupledoperably with the batch container to know, as the batch container isbeing filled, the level or volume of the water that has been added tothe container. The sensing system and method is used to detect when thefilling of the purified water and the preparation of the dialysate hasbeen completed. It is thereafter used to know how much of the mixeddialysate has been used during a single therapy or over multipletherapies.

In a further dialysis example, bagged dialysis fluid is provided, e.g.,for PD. First and second bagged dialysates can be provided having firstand second different dextrose levels, and thus different conductivities.The first and second dialysates can be administered sequentially or bemixed in a container to form a blended dialysate having a dextrose leveltailored for the patient. The container may also be used as a containerfor heating and/or weighing of the dialysis fluids. The present leveland volume detection system and method is provided to know how muchdialysate is present (during filling and draining to the patient) in themixing/heating container regardless of whether only the first dialysateis pumped to the container, only the second dialysate is pumped to thecontainer, or a blend of the first and second dialysates is pumped tothe container.

In another PD implementation, bagged dialysis fluid components areprovided and are mixed in a container to form an overall mixed dialysatethat is delivered to the patient. The present level and volume detectionsystem and method is provided to know how much of each component isadded to the container to achieve a desired mix ratio. The system andmethod can then be used to know how much of the mixed dialysate remainsin the tank during the course of therapy. The tank as above can befurther used to heat and/or weigh the mixed dialysate.

The present system and method are not limited to renal failure therapiesand can be used with any type of medical fluid delivery. In a druginfusion example, it the drug infusion can include multiple liquid orliquefied drugs that are delivered to the patient sequentially or in acombined manner. In another example, supplies of multiple constituentsof a drug may be blended at the time of use, e.g., if the mixed drug isunstable if stored over a period of time. The different drugs andconstituents can have different conductivities. A container is providedin use with the level or volume sensing system and method of the presentdisclosure, such that the volume or level of any drug, any constituent,or any combination of drug or constituent can be sensed, regardless ofconductivity, and regardless of whether the conductivity is static orchanging.

In one embodiment, the tank level or volume sensing system includes anelectrically insulating substrate onto which a pair of radio frequency(“RF”) probes or electrodes is applied. The substrate can for example bemade of an FR-4 or printed circuit board material, a plastic material, aglass or ceramic material, a polyimide material, or be a combination ofany of the above. The electrodes, which can be copper, aluminum, nickel,lead, tin, silver, gold, alloys thereof, and combinations thereof, areplated, adhered, soldered, sputtered, mechanically fixed to theinsulating substrate, or done so using a combination of thesetechniques.

The electrodes are sized and positioned relative to each other so as tobe able to transmit a radio frequency signal, one electrode being thesignal or emitting electrode, the other electrode being the receiving orground electrode. The electrodes extend along the substrate, e.g.,vertically, a distance corresponding to a full level of the tank orcontainer. The electrodes can for example be about 12 mm (0.47 inch)wide and be spaced apart from each other about 2 mm (0.08 inch) to about8 mm (0.31 inch), although other distances may be obtainable. Thethickness of the electrodes can be a standard application thickness forwhatever application process is used for applying the electrodes to thesubstrate, for example, about 100 micrometers. The substrate is fixedclose to the tank or container, for example, about 2 mm (0.08 inch) fromthe container. As described below, the dimensions of the sensors may beoptimized to improve system reliability.

In an alternative embodiment, the electrodes are applied directly to theoutside of the tank or container. Here, the additional substrate is notneeded. In either case it should be appreciated the electrodes do notcontact the medical fluid and therefore cannot contaminate the fluid.

The electronic circuitry can be provided in whole or in part on thesubstrate or on a separate circuit board located with the othercontrollers and circuit boards of the medical fluid machine, forexample, in a safe area of the machine for housing electronics. Theelectronics can include, for example, an oscillator that oscillates orgenerates a radio frequency signal. The signal can be a low powersignal, e.g., on the order of −10 dBm (the power ratio in decibels ofthe measured power referenced to one milliwatt). The RF signal isamplified and then sent to the signal or emitting probe of the sensor.The signal may optionally be matched before it is sent to the signalprobe of the sensor. The receiving or ground probe of the sensor picksup the transmitted RF signal and then returns the signal to ground.

As the signal travels from one probe to the other, an electric field(“EF”) is generated, which travels from the positively charged probe tothe negatively charged probe. An RF wave propagates between the twometal electrodes along the side of the tank and perpendicular to thedirection of the electric field. The impedance that the RF transmissionsees or is subjected to when traveling from the signal electrode to thereceiving electrode changes based upon the amount of medical fluidthrough which the electric field has to pass. The RF wave passes throughan unchanging medium, such as air, on the side of the sensor substratefacing away from the tank. On the side of the sensor substrate facingthe tank, however, the RF wave passes through a changing combination ofair and medical fluid. The more full the tank or container, the moreliquid the wave sees along its transmission path.

As described in additional detail below, the two strips of conductivematerial fixed to the side of the tank can be operated as a transmissionline. In one model of the equivalent transmission line, the water in thetank represents the load of the transmission line. As the water levelchanges, the overall load seen by the transmission line also changes,changing the overall impedance. Thus, measuring a change in impedanceallows determining the water level in the tank.

In one embodiment the system senses the overall impedance seen by thetransmission line. The sensed impedance is governed by the equation:

${Z_{o} = {\sqrt{\frac{\mu\; o}{ɛ}}{F(g)}}},$whereF(g) is a function of the geometry of the electrodes, μ_(o) is thepermeability of free space, and ∈ is an equivalent dielectric of theoverall medium through which the RF signal must pass when travelingalong the transmission line. The equivalent dielectric ∈ can becharacterized as follows:

${ɛ \approx \frac{{ɛ\; o} + {ɛ\; d}}{2}},$where∈₀ is the dielectric constant of free space and ∈_(d) is the dielectricconstant of water, dialysate, drug, medicament or other water-basedmedical fluid. When the liquid level in the tank or container changes ∈,the equivalent dielectric, changes accordingly, affecting the sensedimpedance Z_(o) according to the equation above. Applicants havesuccessfully tested the tank level or volume sensing system of thisdisclosure. Data for the tests is illustrated below.

As is known to one of skill in the art, the impedance is made up of theresistance and the phase as shown by the following equation:Z=R+jX, where

Z is the overall impedance,

R is the resistance, and

X is the phase seen by the transmission line. Depending on circuitryused with the sensor, the water level or volume in the tank can bedetermined based not only on the impedance, but also on the resistanceor the phase seen by the system.

In one embodiment the system senses the resistance seen by thetransmission line. The electronics in this embodiment includes aresistance sensing circuit that measures the resistance along theelectrical line leading from the amplifier to the signal electrode(i.e., measures the real part of the sensor's input impedance as opposedto its reactance). An output of the resistance sensing circuit isconverted to a duty cycle, amplified, digitized and then sent to amicroprocessor and associated memory to be analyzed and converted into atank level value or a tank volume value. The microprocessor andassociated memory, like the other electronics of the tank level orvolume sensing system, can be located locally at the sensor substrate orremotely with the other electronics of the medical machine. The samemicroprocessor and associated memory in one embodiment controls the RFoscillator and receives the digitized duty cycle resistance output.

As discussed herein, the RF sensing system may include tuningelectronics, including for example capacitors and inductors, that aresized to minimize, as much as possible, the output of the impedance thatwould be affected by changes in conductivity. The sensor output may beshifted to minimize or zero out the reactive or imaginary part of theimpedance, so that the output consists largely of the real or resistivepart of the impedance. By doing so, the sensing system is advantageouslyunaffected by the conductivity of the medical fluid for certainfrequencies, allowing different fluids to be delivered to the tank orcontainer at different times, and allowing each fluid to be sensedaccurately and repeatably.

In another embodiment, the sensing system of the present disclosurelooks instead to the electrical phase shift that also accompanies thechange in impedance due to the overall change in the dielectric. Indescribing this embodiment, it is important to understand the differencein physical length of the transmission lines or electrodes and an“electrical length” associated with the changing dielectric. Theelectrical length is proportional to the physical length of thetransmission line or electrodes. In particular, the electrical lengthcan be expressed as a function of the physical length L of thetransmission line, as follows:electrical length=∈*β₀ *L, where

∈ is the overall dielectric constant,

β₀ is the wave number in free space, and is a constant, and

L is again the physical length of the transmission line or sensor, andis a constant.

Overall dielectric constant ∈ as shown in the equation above is afunction of ∈₀, the dielectric constant of free space, and ∈_(d), thedielectric constant of water, dialysate, drug, medicament or otherwater-based medical fluid. When the liquid level in the tank orcontainer changes, the electrical length also changes according to theequation:change in electrical length=2∈₁β₀ L−2∈₂β₀ L, where

∈₁ is the overall equivalent dielectric constant at liquid level 1, and

∈₂ is the overall equivalent dielectric constant at liquid level 2.

The tank and electrodes used for the resistance sensing embodiment,impedance sensing embodiment or phase shift embodiment can be the exactsame structures. The circuitry for the resistance, impedance, or phaseshift embodiments, which can be implemented and located in any of themanners described herein, will be different. In one embodiment, thephase shift circuitry described in detail below uses a frequency mixerthat outputs a direct current (“DC”) signal indicative of phase shift.

The phase shift due to changing dielectric is not affected by fluidconductivity. Unlike the resistance system or impedance system, whichmay need to be tuned so as not to be affected by fluid conductivity, thephase shift system is inherently unaffected by fluid conductivity. Thus,tuning circuitry is not needed with the phase shift system, which isadvantageous.

Based on the foregoing and following description, it should beappreciated that it is an advantage of the present disclosure to providea tank or container level or volume sensing system that is robust.

It is another advantage of the present disclosure to provide a tank orcontainer level or volume sensing system that is relatively inexpensive.

It is a further advantage of the present disclosure to provide a tank orcontainer level or volume sensing system that is accurate andrepeatable.

It is yet another advantage of the present disclosure to provide a tankor container level or volume sensing system that provides information inreal time.

It is yet a further advantage of the present disclosure to provide atank or container level or volume sensing system that is non-invasive,allows for a hermetically sealed container, and does not require directsensing contact with the sensed fluid.

Moreover, it is an advantage of the present disclosure to provide a tankor container level or volume sensing system that is inherentlyunaffected by fluid conductivity, such that different medical fluids ordifferent components thereof can be sensed at different times.

It is still a further advantage of the present disclosure to provide atank or container level or volume sensing system that is compatible withmedical fluid mixing, medical fluid preparation, and medical fluiddelivery.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic block diagram of one embodiment of the tank orcontainer level or volume sensing system of the present disclosure.

FIG. 2 is a front elevation view of one embodiment for the tank orcontainer level or volume sensor of the present disclosure.

FIG. 3 is a schematic view showing the electric field generated from thesignal electrode of the sensor to the receiving electrode of the sensor.

FIG. 4 is an electrical schematic of one embodiment of the tank orcontainer level or volume sensing system of the present disclosure.

FIG. 5 is a front view of one alternative embodiment for a control boardof the sensor of the present system and method.

FIG. 6 is a front elevation view of another embodiment for the tank orcontainer level or volume sensor of the present disclosure.

FIG. 7 is an electrical schematic of another embodiment of the tank orcontainer level or volume sensing system of the present disclosure.

FIG. 8 is a graph illustrating results from testing of the RFtransmission level or volume sensing system of the present disclosure.

FIG. 9 is a plot of impedance versus tank liquid level for two liquidsat two different conductivities showing that the impedance output of thesensor of the present disclosure is relatively insensitive toconductivity.

FIG. 10 is a plot of tank level/volume over time showing the accuracy ofthe output of the system and sensor of the present disclosure comparedwith expected tank level/volume data.

FIG. 11 is an electrical schematic of yet another embodiment of the tankor container level or volume sensing system of the present disclosure.

FIG. 12 is a plot of phase shift output versus tank liquid level forliquids at multiple different conductivities showing that the phaseshift output of the sensor of the present disclosure is unaffected byconductivity.

FIG. 13 is a graph of a frequency sweep performed when the tank isempty.

FIG. 14 is a graph of a frequency sweep performed when the tank is full.

FIG. 15 is a graph of the frequency sweeps of FIGS. 13 and 15 plottedtogether to determine an operating frequency.

FIG. 16 is a schematic diagram illustrating any of the tank or containerlevel or volume sensors discussed herein operating with an online bloodtherapy treatment system.

FIG. 17 is a schematic diagram illustrating any of the tank or containerlevel or volume sensors discussed herein operating with a batch orsemi-batch blood therapy treatment system.

FIG. 18 is a schematic diagram illustrating any of the tank or containerlevel or volume sensors discussed herein operating with baggedperitoneal dialysis system.

FIG. 19 is a schematic diagram illustrating any of the tank or containerlevel or volume sensors discussed herein operating with medical fluidmixing system.

DETAILED DESCRIPTION

Referring now to the drawings and in particular to FIG. 1, a systemdiagram for one embodiment of the container level or volume sensingsystem 10 is illustrated. System 10 includes a microprocessor andassociated memory 12, which can be a delegate processor thatcommunicates with a master or primary processor of a medical device intowhich system 10 is installed. Microprocessor and associated memory 12can be located locally with the sensor of system 10. Alternatively,microprocessor and associated memory 12 are located remotely from thesensor of system 10, for example, in a safe place away from the fluidcomponents of the medical fluid system. Microprocessor 12 cancommunicate with one or more memories to provide the functionalitydiscussed herein.

In FIG. 1, microprocessor and associated memory 12 controls avoltage-controlled oscillator (“VCO”) 14. VCO 14 in an embodiment isselected to produce an alternating signal, which can be in a radiofrequency range, e.g., from about thirty kilohertz to aboutthree-hundred gigahertz. VCO 14 can be an off-the-shelf component, whichcan produce the radio frequency as a sine wave or square wave.Oscillator 14 is powered via a voltage V_(cc), which can for example be5V. Voltage V_(cc) can be obtained from a power source located oncontrol board 100 or from a power source located elsewhere. Oscillator14 is also connected to ground 70, which can be a system ground or earthground. Power amplifier 16 is powered via a separate power supply theV_(dd), which can for example be 5V. Power amplifier 16 is also taken toground 70. The amplified RF signal from power amplifier 16 is sent via acoaxial wire to a connection point that is connected, for example via awire-carrying ribbon or cable to a signal electrode of sensor 20, whichoperates as a transmission line as described below.

The RF signal from VCO 14 is amplified via a power amplifier (“AMP”) 16.In one embodiment, the RF signal from VCO 14 is a very low power signal,e.g., in the range of about −20 to 5 dBm (the power ratio in decibels ofthe measured power referenced to one milliwatt). AMP 16 or an attenuatoramplifies or attenuates the RF signal to about −10 dBm.

The amplified RF signal emanating from AMP 16 is then sent to sensor 20,which is discussed in detail herein. In general, however, sensor 20 ispositioned operably adjacent to or onto a medical fluid tank 40, asshown in detail below. Sensor 20 in essence opens or allows the electricfield associated with the RF signal to travel through the medical fluidtank 40. The ability of the electric field associated with the RF signalto travel through the medical fluid tank 40 is dependent upon how muchfluid resides in tank 40. That is, the impedance to the RF signaltransmission along transmission line sensor 20 is dependent upon howmuch liquid resides within the tank 40. Accordingly, a level sensingcircuit 60 is provided to determine the overall impedance that sensor 20sees. The level sensing circuit 60 may also determine the resistance orphase shift that sensor 20 sees.

By measuring the impedance seen by the electric field associated withthe RF signal passing through container or tank 40, the sensing system10 can determine the level of medical fluid within tank 40. By knowingthe geometry of tank 40, the level sensing circuit therefore alsoenables a volume of liquid within tank 40 to be determined accuratelyand in real time. The impedance measured from sensor 20 is compared to areference value, yielding a duty cycle that can be digitized by ananalog-to-digital (“A/D”) converter 62. The sampling rate can be variedfrom milliseconds to several seconds depending upon the need.

Delegate processor 12 reads digitized signal 62 and, for example incooperation with one or more memories, converts the digitized signalinto a value corresponding to tank or container fluid level or volume.It is contemplated that in a therapy or fluid to patient deliverysituation, processor 12 queries a control board for system 10 for animpedance reading every so often, for example, every minute, to know andhelp maintain a desired fluid level in essentially real time. In amixing situation, readings can be taken much more frequently, forexample, on the order of milliseconds.

Referring now to FIG. 2, one embodiment for sensor 20 is illustrated. Inthe illustrated embodiment, sensor 20 is positioned directly adjacent toa fluid tank or container 40. Tank or container 40 is illustrated asbeing generally rectangular, having a front wall 42, top wall 44 andside wall 46. It should be appreciated however that container 40 canhave other shapes as desired. In an embodiment, container 40 includes atleast one relatively flat surface, such as front wall 42, adjacent towhich sensor 20 is located. Otherwise, the other surfaces, such as topsurface 44, side surface 46, and the back surface of container 40 canhave projections, undulations, be rounded, curved or otherwisenon-planer. In the illustrated embodiment, top wall 44 includes an inletport or connector 52 for fluid-tightly receiving a fluid inlet tube forthe medical device and a fluid outlet port or connector 54 forfluid-tightly receiving a fluid outlet tube of the medical device orsystem. In one embodiment, the RF wave propagates in a directionperpendicular to the top wall 44.

Sensor 20 includes a substrate 22, which can be made of any suitablenon-conducting material, such as FR-4 material, ceramic, plastic, apolyimide, glass and any combination thereof. Substrate 22 includes asignal electrode 24 and a ground electrode 26. Electrodes 24 and 26 aremade of a suitable conductive material, such as copper, nickel, gold,silver, lead, tin and alloys and combinations thereof. Electrodes 24 and26 are electrochemically, photo-chemically, and/or mechanically plated,adhered, soldered, sputtered or sprayed onto substrate 22. For example,electrodes 24 and 26 can be copper electrodes formed on an FR-4 materialvia a known photo-etching process, which can provide a very detailedshape and geometry for electrodes 24 and 26.

In an embodiment, electrodes 24 and 26 have the same length or lengththat is very close to the length L corresponding to a full liquid levelwithin container 40, which may or may not be the largest verticaldimension of the container. That is, substrate 22 is positioned relativeto container 40, such that the bottom of electrodes 24 and 26 arealigned with the bottom of container 40, while the top of electrodes 24and 26 are aligned with the top 44 of container 40. Alternatively, if afull level within container 40 is some level below top 44, then the topof electrodes 24 and 26 are instead lowered to this full level.

Electrodes 24 and 26 are shown facing outwardly from substrate 22 andcontainer 40. Alternatively, electrodes 24 and 26 can be placed on theinside of substrate 22 so as to be located between substrate 22 andcontainer 40. In either case, it is contemplated to space substrate 22and electrodes 24 and 26 very close to surface 42, such as from aboutone-half millimeter (0.02 inch) to about five millimeters (0.197 inch).The widths of electrodes 24 and 26 can be varied as desired to provide ageometry that functions well with the RF signal generated from the VCO14. One example width range is about a half centimeter (0.196 inch) to1.5 cm (0.6 inch) for each of electrodes 24 and 26. The electrodes 24and 26 can be spaced apart from each other by the approximate width ofelectrodes or some distance less than the electrode widths, such asbeing spaced apart one centimeter or less. The application thickness ofelectrodes 24 and 26 can be the standard application thickness forwhatever process is used to form the electrodes. For example, anapplication thickness of about 20 to 100 micrometers is suitable in oneembodiment.

As discussed below, sensor 20 is calibrated so as to yield a look-uptable that processor 12 uses to correlate signal data to fluid leveland/or fluid volume. When the geometry (e.g., size and/or spacing) ofelectrodes 24 and 26 is modified, the calibration needs to be performedagain. It is contemplated to check the calibration for the particulargeometry of electrodes 24 and 26 at the beginning of a pre-therapy(e.g., prime), therapy or post-therapy (e.g., rinseback or disinfection)procedure that uses system 10 and sensor 20. Here, a known quantity ofthe liquid is delivered to container 40. The reading taken by system 10is compared to the known quantity or its associated fluid level. If thereading agrees with or is only slightly different than the knownquantity, the calibration is maintained and used for the procedure. Ifdisagreement between the reading and the known quantity is great, thenliquid is either removed from or added to container 40 and thecomparison procedure is repeated. If after a number of calibrationchecks it is found that the reading is off by a consistent delta, eachvalue of the calibration look-up table is modified by the consistentdelta and a modified calibration is used for the procedure. If after anumber of calibration checks it is found that the reading is off by anon-consistent delta, processor 12 can take an average of the deltas andmodify each of the values of the calibration look-up table by theaverage delta to form a modified calibration that is used for theprocedure. If the reading is found to vary an unacceptable amount fromthe known quantity, then processor 12 causes the medical device to postan alarm.

In the illustrated embodiment, substrate 22 is mounted to studs 48extending from surface 42 of container 40, so as to be set precisely atgap distance G. In an alternative embodiment, substrate 22 is mounted toa fixed portion of the medical machine independent of container 40 orthe fixture for container 40. In the illustrated embodiment, substrate22 can be provided with apertures 28 that snap fit into groovespositioned precisely along studs 48 of container 40, so as to set a gapdistance G that does not vary even if container 40 is vibrated or movedslightly during the functioning of the medical device.

Electrodes 24 and 26 in the illustrated embodiment terminate at aterminal block 30 located, for example, below the bottom of container40. Terminal block 30 connects to a protective ribbon or cable 34, whichcan for example be flexible and insulating, and which in turn leads tocoaxial line 102 and ground line 104 of control board 100 shown in FIG.4.

A protective coating, such as a conformal or epoxy coating 32 is sprayedor laminated over electrodes 24 and 26 and elsewhere along substrate 22and terminal block 30 as needed to ensure that the wetness, humidity andheat potentially generated within the medical device does not harm ordegrade the components and/or performance of sensor 20 and system 10.Substrate 22 in an alternative embodiment is made part of or placedwithin a protective housing (not illustrated), such as a protectiveplastic housing.

Referring now to FIG. 3, the operation of electrodes 24 and 26 of thesensors of FIG. 2 is illustrated. FIG. 3 is a top view of a container40. Here again, electrodes 24 and 26 are placed adjacent to or on frontsurface 42 of container 40. When the energy source is activated, anelectric field is created between signal electrode 24 and groundelectrode 26, in which the electric field lines travel from the positiveemitter 24 to the negative receiver 26. The direction of the electricfield is parallel to the top surface of a fluid in one embodiment. Inother words, the direction of the electric field is parallel to a planedefining the level of the fluid in the tank. The electric field carriesan RF signal, in which the propagation of the RF wave is perpendicularto the direction of the electric field lines. Thus the electrical fieldis generated from positive electrode 24 to negative electrode 26, andthe RF wave propagates along the electrodes.

The electric field carrying the RF signal travels from signal electrode24 to ground electrode 26 in two directions as shown in FIG. 3, namely,through a first path along the outside of container 40 and through asecond path along the inside of container 40. Thus, a portion of theelectric field travels through a fluid and the remainder of the electricfield travels through free space. As the fluid level inside the tank islowered, more and more of the electric field travels through free space.In one embodiment, as less of the electric field is covered by thefluid, the effective load of the modeled transmission line changes.

The path along the outside of container 40 is modeled in the illustratedexample as being solely through air, which has a dielectric constant offree space, ∈₀ equal to one. Assuming for sake of example that container40 is completely full of liquid water, the path for the RF signal on theinside of container 40 is solely through a medium having a dielectricconstant of water, ∈_(d) equal to approximately 80. It is assumed thatany fluid that will reside within container 40 is either water or asubstantially water-based medical fluid, such as dialysate forhemodialysis and peritoneal dialysis, replacement fluid forhemofilitration and hemodiafiltration, and any operating room drug orliquid, such as saline.

There is then an equivalent dielectric, ∈, which is equal to ∈₀ plus∈_(d) divided by two. Thus if tank 40 is completely empty, a dielectricenvironment ∈₀ for air will exist on the outside and the inside of thetank, resulting in an equivalent dielectric being one plus one dividedby two, which equals one, the dielectric constant of free space. Aswater or medical fluid fills container 40, the dielectric constant ofair is continuously replaced by more and more media having thedielectric constant of water, until container 40 is completely full, atwhich point the equivalent dielectric ∈ is equal to approximately(1+80)/2 or 40.5. In this manner, the characteristic resistance of thetank imparted to the RF sensor varies as a result of the effectiveequivalent dielectric varying anywhere from 1 to 40.5.

As the water or medical fluid level changes, the overall dielectricconstant also changes. Specifically, as the fluid level inside of thetank decreases, the overall dielectric constant, which can beapproximated as averaging the dielectric constant inside the tank plusthe dielectric constant outside the tank divided by two, also decreases.When the overall dielectric constant decreases, the impedance measuredby the circuitry attached to the sensor also decreases.

The characteristic impedance Z₀ varies according to the formula

${Z_{0} = {\sqrt{\frac{\mu\; o}{ɛ}}{F(g)}}},$whereμ₀ is the permeability of free space, which is a constant, and F(g) is afunction of the geometry of the RF transmission line, includingelectrodes 24 and 26. The electrodes are accordingly sized and shaped tobe optimized via their geometry to provide a changing output having adesirable accuracy, linearity, repeatability and robustness for aparticular geometry of tank 40.

The system is calibrated so that it is known how much fluid correspondsto an impedance. Once the transmission line has been established andcalibrated, the electronic circuitry attached to the sensor can monitorvarious parameters to determine the level of the tank. As describedabove, the two electrodes can be used to model a transmission line. TheRF wave sent into the transmission line, i.e., the incident wave,propagates along the transmission line and is then affected by the loador impedance of the tank. The wave reflects back, creating the reflectedwave, and the difference in characteristics between the incident waveand the reflected wave can be used to determine the water level of thetank. The difference in characteristics between the two waves depends onthe equivalent dielectric constant at that water level. In other words,a change in water level leads to a change in the equivalent dielectricconstant, which leads to a change in the measured values of the incidentand reflected waves.

In one embodiment, the electrodes can model a slot line transmissionline. Construction of a slot line transmission line is well known in theart. See, for example, Holzman, Essentials of RF and MicrowaveGrounding, p. 60 (2006); Gupta, Microstrip Lines and Slotlines, 2ndEdition, Artech House Microwave Library, pp. 269 to 340 (1996); S. B.Cohn, Slot-line—An Alternative Transmission Medium for IntegratedCircuits, IEEE G-MTT International Microwave Symposium Digest, pp. 104to 109 (1968), which are incorporated herein by reference.

Parameters that affect whether the electrodes operate as a reliable slotline transmission line include, for example, the frequency of thesystem, the distance of the electrodes from the tank, the width andthickness of the electrodes, and the distance between the electrodes.

Referring now to FIG. 4, a control board 100 provides an exampleembodiment of level sensing circuit 60 that can be used in system 10that correlates the resistive component of impedance to a water fluidlevel. Control board 100 includes components 12, 14, 16 and 62 (notshown) and connects to sensor 20 (not shown). In particular, controlboard 100 includes oscillator or VCO 14, which is connected via acomputer link to processor 12 as show in FIG. 1. Although notillustrated in FIG. 4, system 10 in one embodiment provides a filter,such as a low pass filter, that removes the DC component from theamplified signal. Control board 100 also includes a return or groundconnection point connected to sensor 20 via the cable or ribbon. Thereturn or ground connection point runs via a return line 104 to ground70. The filter is also taken to system or earth ground 70.

It is contemplated to locate control board 100 in a safe electronicsarea of the medical fluid device, for example, away from potentialsplashing and heat generated within the medical fluid machine. It isfurther contemplated to locate microprocessor and associated memory 12of system 10 on a board that is separate from control board 100. Thatis, processor 12 could be a remote microprocessor that controls otherfunctions of the medical device, for example, other functions related tothe pumping of medical fluid to and from the medical fluid tank 40(FIGS. 2 and 6).

Control board 100 of FIG. 4, which is one embodiment of the levelsensing circuit 60 of FIG. 1, is tapped off of coaxial wire 102 of FIG.2 (not shown). Control board 100 analyzes the resistance along coaxialwire 102 via components 64 a, 64 b and 64 c. Components 64 a, 64 b and64 c are capacitors and inductors that can be tuned during calibrationto minimize the impact from the imaginary part, or reactance, of theimpedance. In an experiment for sensor 20, the results of which arediscussed below with FIGS. 8 to 10, level sensing circuit 60 of controlboard 100 was structured such that capacitor 64 a had a value of 2.7 pF,capacitor 64 b had a value of 5.6 pF and inductor 64 c had a value of1.8 nH. Using these values for the capacitors and inductors, a tuningcircuit operating at 1.4 GHz drove the reactive part of the impedance tozero effectively. These values are merely examples of suitable valuesand are non-limiting. That is, other values, and perhaps other types oftuning circuits may be used for the same or different frequency.

A comparator (not shown) is provided which compares the output of levelsensing circuit 60 to a reference signal. The comparator is powered viathe same voltage V_(dd) that powers power amplifier 16. Voltage V_(dd),like voltage V_(cc), can be obtained from a source located on controlboard 100 or from a source located elsewhere within the medical device.The signal from the comparator is digitized via analog-to-digitaldigital converter 62, shown above in FIG. 1. The digitized signal issent to processor 12, as has been discussed herein. Processor 12operates with one or more memories that in one embodiment store apre-loaded or pre-stored look-up table that has been created from aprior calibration of sensor 20. Processor 12 uses the look-up table tomatch the digitized instantaneous or real-time signal to a correspondingtank fluid level. Since the geometry of the tank is known, any tankfluid level can be correlated to a tank fluid volume. In this manner,processor 12 can alternatively or additionally match the instantaneousor real-time signal to a tank fluid volume.

Referring now to FIG. 5, an alternative control board 120 isillustrated. Here, the electronics associated with control board 100(FIG. 4) are located on the same insulative substrate 122 along withsignal electrode 24 and ground electrode 26 of FIG. 2. Substrate 122 canbe formed of any of the materials discussed above for substrate 22.Electrodes 24 and 26 can be of any of the materials, have any of thegeometries and be applied in any of the manners discussed above forelectrodes 24 and 26 of FIG. 2. In control board 120, components such asVCO 14, power amplifier 16, filter, level sensing circuit 60, A/Dconverter 62 and comparator described above are also located on the sameboard as sensing elements 24 and 26. In an embodiment, the componentscommunicate electrically via traces photo-etched onto substrate 122.

The respective components shown in FIG. 4 running to ground 70 are alsoshown in FIG. 5 running to ground terminals 70, which are then connectedto system or earth ground when control board 120 is plugged into place.Likewise, a voltage terminal V_(dd) is plugged into a voltage line thatpowers the comparator and power amplifier 16. Still further, a voltageterminal V_(cc) is connected to a voltage source powering VCO 14. Aconformal, epoxy or other suitable protective coating 32 is applied overthe electrodes and electrical components of control board 120 as neededto protect such components from the operating conditions residing withinthe medical device. Besides the ground and voltage connection, VCO 14and A/D converter 62 are configured to communicate with processor 12(located for example on a remote processing board) via data signalconnectors 124, such as universal serial bus (“USB”) connectors.

Referring now to FIG. 6, a further alternative configuration for thesensor 20 of system 10 is illustrated. In the illustrated embodiment,electrodes 24 and 26 are applied directly to surface 42 of container 40via an adhesively coated protective film 132, so as to eliminate theseparate substrate 122. Adhesive film 132 can be any suitablenon-conducting and water impermeable film, such as a plastic or polymerfilm. Signal electrode 24 and ground electrode 26 are applied first tothe sticky or adhesive side of film 132. Film 132 is then taped tosurface 42 of container 40. Alternatively, electrodes 24 and 26 areapplied first to surface 42, followed by protective film 132.

Film 132 extends with electrodes 24 and 26 so that the electrodesconnect electrically to a terminal connector 30, which in turn connectsfurther to cable or ribbon 32 that extends leads to coaxial line 102 andground line 104 located safely within the medical device machine atcontrol board 100. In an embodiment, the wall thickness of surface 42 issized to ensure that electrodes 24 and 26 are spaced an adequatedistance away from the interior of container 40. Alternatively, ifneeded, spacers, such as non-conductive spacers, can be used to setelectrodes 24 and 26 away from the interior of container 40.

It is contemplated in any of the sensor configurations discussed hereinto locate electrodes 24 and 26 away from other conductive materialswithin the medical device a certain distance, such as about onecentimeter (0.394 inch) or more. The spacing helps the operation of thesensor, discussed above.

FIG. 7 is a circuit diagram of an example control board 300 used withthe sensor 20 of system 10 for determining a fluid level using theimpedance seen by an RF wave. Control board 300 includes components 12,14, 16 and 62 (not shown) and connects to sensor 20 (not shown). Inparticular, control board 300 includes a microprocessor (FIG. 1) andassociated memory 12, which control a voltage-controlled oscillator(“VCO”) 14. Control board 300 operates with sensor 20, which can beimplemented in any of the configurations discussed above, e.g., withFIG. 2, 5 or 6. Electrodes 24 and 26 (not shown) are connectedelectrically to the circuitry of control board 300. The electricalconnection of electrodes 24 and 26 from sensor 20 to control board 300can be by flexible cable, insulated wire, or via any of the waysdiscussed herein for system 10.

It is contemplated (as with control board 100) to locate control board300 in a safe electronics area of the medical fluid device, for example,away from potential splashing and heat generated within the medicalfluid machine. It is further contemplated to locate microprocessor andassociated memory 12 of system 10 on a board that is separate fromcontrol board 300. That is, processor 12 could be a remotemicroprocessor that controls other functions of the medical device, forexample, other functions related to the pumping of medical fluid to andfrom the medical fluid tank 40 (FIGS. 2 and 6). In an embodiment,control board 300 can be implemented on a printed circuit board (“PCB”,e.g., FR-4 or other type listed above), which can be about two inches bytwo inches (e.g., about five cm by five cm).

Control board 300 includes frequency mixing circuitry 324, whichgenerates an electrical, e.g., millivolt (“mV”) output, which isparticular for a certain water level. In one implementation, RF and LOports on mixing circuitry 324 are used for the forward, or incident,signal 326 and the reflected signal 328, respectively, frombi-directional coupler 330, and an IF port on the mixing circuitry 324is used as the sensed output. Because the frequencies of the forwardsignal 326 and reflected signal 328 are the same, and the forward phaseis constant, the output from the IF port results in phase information ofthe reflected signal, which can be used to determine the liquid levelwithin tank 40.

Oscillator 14 generates a sinusoidal signal at a desired and selectedfrequency as the RF input on control board 300. The signal istransmitted through bi-directional coupler 330 to the transmission line(not shown). The bi-directional also outputs an incident signal as wellas a reflected signal. The incident signal includes a magnitude and aphase for the signal going from the VCO 14 to the probes 24 and 26.

The signal transmitted to the transmission line will be reflected inpart due to the impedance of the tank 40 as well as any mismatch of thetransmission line. As is known in the art, impedance matching circuitrycan be used to reduce impedance mismatching, such as, for example, byshifting impedances to the real axis of a Smith chart as much aspossible. In one embodiment, any reflection due to a mismatch is ignoredor eliminated, leaving only the reflection due to the impedance of tank40. The reflected signal 328 can be measured at the reflected signaloutput of bi-directional coupler 330. The reflected signal includes amagnitude and a phase for the signal reflected from the probes 24 and26. In the illustrated embodiment, the incident signal 326 is then splitinto magnitude and phase results by splitter 336, and reflected signal328 is split into magnitude and phase results by splitter 338. Powerdetectors 332 and 334 are used to measure the magnitudes of the incidentsignal 326 and reflected signal 328, respectively. Phase detectorcircuit 324 (frequency mixer) can measure the phase difference betweenthe incident and reflected signals.

A network analyzer, such as for example, ENA Series Network Analyzer(E5071C), can be used to analyze S-parameters that provide informationabout the incident signal, reflected signal, and phase shift. TheS-parameters are expressed in terms of magnitude and the phase, wherethe splitters separate the magnitude and phase results, providingmagnitude results to the power detectors and providing phase results tothe phase detector. The power detectors 332 and 334 convert magnituderesults into voltages representing the magnitudes of the incident andreflected signals, respectively. The ratio of the magnitudes of theincident signal to reflected signal can be used to determine areflection coefficient. Phase detector circuit 324 converts phaseresults into a voltage representing the difference in phase between theincident signal and the reflected signal.

With measurements of the reflection coefficient and the phase shift, theimpedance seen by the transmission line can be determined. The impedancecorresponds to a level of medical fluid in the tank, and thus medicalfluid level can be determined. This operation can be confirmed with aSmith chart, where a reading of the reflection coefficient and phaseangle can be correlated to an overall impedance.

The components used during experimentation are readily available and canbe implemented as described herein by those skilled in the art. Forexample, component-provider Minicircuits manufactures bi-directionalcouplers (e.g., model number ZX30-20-20BD+), power detectors (e.g.,model number ZX47-50LN+), VCO (e.g., model number ZX95-1600W+), phasedetectors (e.g., model number ZFM-5X+), and power splitters (e.g., modelnumber ZX10R-14+) that may be used in control board 300 of FIG. 7.

FIG. 8 illustrates results from preliminary testing of system 10. FIG. 8shows a roughly linear change in impedance over a liquid level change offive-hundred millimeters. It is believed that the results can be madeeven more linear and repeatable by experimenting with differentgeometries and materials for electrodes 24 and 26. Further, a constantcross-sectional tank will likely yield more linear results. It shouldalso be appreciated that the impedance change in FIG. 8 is the steepest,and thus the most sensitive, in the middle of the curve, flattening outat the beginning and end of the curve. It is contemplated then to matchthe expected high and low levels of the fluid within tank 40 with themiddle or steep range of the sensor output curve shown in FIG. 8. Inthis manner, the liquid level is likely to vary within the mostsensitive range of sensor 20.

It is accordingly contemplated to store in processor 12 and itsassociated memory a database correlating a particular impedance,resistance or reactance to a particular liquid level within a water ormedical fluid holding tank, such as tank 40. The correlation table isdetermined for the tank and sensor electrode geometries. It should beappreciated that, assuming the correlation table takes into account thefull range from completely empty to completely full, a medical devicecan look at the tank level at any desired time and receive anup-to-date, real-time indication of liquid level. And as has beendescribed herein, especially in certain medical applications, it isadvantageous that the system 10 level sensing is not dependent on liquidconductivity.

Test data can be used to confirm that the impedance output of the sensoris relatively insensitive to conductivity. A propagation constant forthe RF signal of sensor 20 is a measure of a change undergone by theamplitude of the RF wave as it propagates through the changing tankfluid level. The propagation constant measures change per unit lengthbut is otherwise dimensionless. The propagation constant can beexpressed as follows:

Propagation constant:

$\gamma = {\sqrt{( {R_{conductor} + {j\;\omega\; L}} )( {G + {j\;\omega\; C}} )} = \sqrt{( {\frac{R_{conductors}}{F(g)} + {j\;\omega\;\mu_{0}}} )( {\sigma_{d} + {j\;{\omega ɛ}}} )}}$Such that: L=μ₀F(g) C=∈/F(g) G=σ_(d)/F(g)

The transmission line impedance for sensor 20 can be derived from theabove propagation constant equations as follows:

$Z = {\sqrt{\frac{R_{conductors} + {j\;\omega\; L}}{\gamma}} = {\sqrt{\frac{R_{conductors} + {j\;\omega\; L}}{G + {j\;\omega\; C}}} = {\sqrt{\frac{\frac{R_{conductors}}{F(g)} + {j\;{\omega\mu}_{0}}}{\sigma_{d} + {{j\omega}\; ɛ}}} \cdot {F(g)}}}}$

For the above equation for impedance Z, R_(conductors) can be consideredto be zero (because transmission line electrodes 24 and 26 are goodconductors) and F(g) can be considered to be equal to one for a slotline break between electrodes 24 and 26. The transmission line impedanceZ can then be simplified to:

${Z = {\sqrt{\frac{{j\omega\mu}_{0}}{\sigma_{d} + {j\omega ɛ}}} = {\sqrt{\frac{{{j\sigma}_{d}{\omega\mu}_{0}} + {\omega^{2}\mu_{0}ɛ}}{\sigma_{d}^{2} + {\omega^{2}ɛ^{2}}}} = {\sqrt{\frac{\mu_{0}}{ɛ}} \cdot \sqrt{\frac{{j\mspace{11mu}\tan\;\delta} + 1}{{\tan^{2}\delta} + 1}}}}}},$where tan

$\delta = \frac{\sigma_{d}}{\omega ɛ}$is the loss tangent.

In one example calculation, frequency (f) for RF is taken to be 1.4 GHz.σ_(d) is used for different conductivity settings ranging as follows:(i) 15 mS/cm, (ii) 10 mS/cm, (iii) 5 mS/cm, and (iv) 0 mS/cm. Thedielectric constant ∈=∈_(r) to ∈₀; where ∈_(r)=80 and ∈₀=1, resultingin:

TABLE 1 for σ_(d) = 15 mS/cm → Z = 41.1411 + 4.8846i for σ_(d) = 10mS/cm → Z = 41.6195 + 3.3201i for σ_(d) = 5 mS/cm → Z = 41.9172 +1.6799i for σ_(d) = 0 mS/cm → Z = 42.0183

As illustrated in Table 1 above, which was obtained according to theabove equations using MATLAB® software, no significant change occurs inthe first number (41.1411, 41.6195, 41.9172 and 42.0183), whichcorresponds to the real part of the impedance, or resistance, when thedielectric's conductivity changes from zero to fifteen mS/cm. Per thederived formulas, high frequencies minimize the change in overallimpedance, explaining these results. The second number (4.8846i,3.3201i, 1.6799i and 0i) corresponding to the imaginary part of theoverall impedance, or reactance, may be tuned out of the results asshown via the electronics described above. The resulting transmissionline sensor 20 is accordingly not sensitive to the liquid conductivitychange at least in the range of zero to fifteen mS/cm.

Referring to FIG. 9, an experimental plot for sensor 20 shows that thereis very little difference in the output of the sensor when theconductivity changes from zero to fifteen mS/cm, which is a range thatshould encompass most of the medical fluids discussed herein. It isexpected too that the conductivity could be increased past fifteen mS/cmwithout significantly affecting sensor output. The reason is due to theloss tangent (equation shown above) being small because a high, e.g.,RF, frequency is used. Suitable frequencies for sensor 20 can be betweenone and two GHz.

Referring now to FIG. 10, accuracy results using system 10 and sensor 20are illustrated. The squares represent known or expected tank level orvolume data. The diamonds represent tank level or volume using data fromsystem 10 and sensor 20. As illustrated, the sensor output matches theexpected output very well all the way through the tank volume range ofzero to 1600 milliliters. As should be appreciated viewing the x-axis ofthe plot of FIG. 10, time increments between readings can be selected asdesired to be on the order of minutes, seconds, or even fractions ofseconds.

Referring now to FIG. 11, another embodiment for detecting tank level orvolume non-invasively for a radio frequency or other high frequencysystem is illustrated in control board 400 for system 10. Control board400 determines medical fluid level based on the phase component of theimpedance or change in electrical length. In general, electrical lengthcan be thought of as the length of a transmission line expressed as thenumber of wavelengths of a signal propagating in a medium. The highfrequency or RF waves propagate more slowly in a medium, such asdialysis fluid, blood, dialysate, or a liquid drug, than in free space.

Sensor 20 and medical fluid tank 40 in FIGS. 2, 5 and 6, including allalternative embodiments discussed for sensor 20 (including all physical,structural and implementation alternatives for electrodes 24 and 26) andtank 40 can be used again in phase shift system implemented usingcontrol board 400. The radio frequency transmission model shown above inFIG. 3, which illustrates ∈₀, the dielectric constant of free space, and∈_(d), the dielectric constant of water, is likewise applicable tocontrol board 400.

FIGS. 2 and 6 show that in one embodiment, electrodes 24 and 26 have thesame length L as the height of tank 40, so that any liquid level withintank 40 can be detected. The physical length L of the transmission linesor electrodes 24 and 26 is a component of the electrical length. Inparticular, the electrical length, φ, can be expressed as a function ofthe physical length of the transmission line L, and the dielectric ofthe medium through which it travels as follows:electrical length, φ=∈*β₀ *L, where

∈ is the overall dielectric constant,

β₀ is the number of waves for a given wavelength or frequencypropagation that occurs in free space over a known distance, such as onemeter, and is a constant stored in memory, and

L is again the physical length of the transmission line or sensor, andis a constant stored in memory.

The overall dielectric constant ∈ as shown in the equation above is afunction of ∈₀, the dielectric constant of free space, and ∈_(d), thedielectric constant of water, dialysate, drug, medicament or otherwater-based medical fluid. When the liquid level in tank or container 40changes, the electrical length φ also changes according to the equation:change in electrical length Δφ=2∈₁β₀ L−2∈₂β₀ L, where

∈₁ is the equivalent overall dielectric constant at liquid level 1, and

∈₂ is the equivalent overall dielectric constant at liquid level 2.

The electrical length φ of free space can be calibrated for a particularconfiguration of tank 40 and electrodes 24 and 26, including the spatialrelationship between tank 40 and electrodes 24 and 26 (as illustrated bydifferent electrode mounting techniques shown in FIGS. 2 and 6).

The free space electrical length is stored in the memory of controlboard 400, which can be located in any of the configurations discussedabove for system 10. When the level of liquid changes within tank 40,the overall dielectric changes, causing the electrical length of thesignal propagated from electrode 24 to change. The circuitry aboveprovides an output that is indicative of the new electrical length. Thefree space electrical length is subtracted from the newly sensedelectrical length forming a Δφ. A lookup table is stored in memory thatcorrelates a particular Δφ with a particular tank level or volume. Thus,the tank level or volume can be known at any time and at any levelwithin the tank.

Like control board 300 from FIG. 7, control board 400 in FIG. 11measures the phase angle of the RF wave. Control board 400 includesfrequency mixing circuitry 424, which generates an electrical, e.g.,millivolt (“mV”) output, which is particular for a certain electricallength or level. In one implementation, RF and LO ports on mixingcircuitry 424 are used for the forward, or incident, signal 426 and thereflected signal 428, respectively, from bi-directional coupler 430, andan IF port on the mixing circuitry 424 is used as the sensed output.Because the frequencies of the forward signal 426 and reflected signal428 are the same, and the forward phase is constant, the output from theIF port results in phase information of the reflected signal, which canbe used to determine the liquid level within tank 40.

Oscillator 14 generates a sinusoidal signal at a desired and selectedfrequency as the RF input on control board 400. The signal istransmitted through bi-directional coupler 430 to the transmission line(not shown). The bi-directional also outputs an incident signal as wellas a reflected signal. The incident signal includes a phase for thesignal going from the VCO 14 to the probes 24 and 26.

As described above with respect to control board 300, the signaltransmitted to the transmission line will be reflected in part due tothe impedance of the tank 40 as well as any mismatch of the transmissionline. As is known in the art, impedance matching circuitry can be usedto reduce impedance mismatching, such as, for example, by shiftingimpedances to the real axis of a Smith chart as much as possible. In oneembodiment, any reflection due to a mismatch is ignored or eliminated,leaving only the reflection due to the impedance of tank 40. Thereflected signal 428 can be measured at the reflected signal output ofbi-directional coupler 430. The reflected signal includes a phase forthe signal reflected from the probes 24 and 26. Phase detector circuit424 (frequency mixer) can measure the phase difference between theincident and reflected signals. As described above, phase detectorcircuit 424 converts phase results into a voltage representing thedifference in phase between the incident signal and the reflectedsignal. Control board 400 in FIG. 11 does not however need to measurethe magnitude of the incident and reflected waves, and instead relies onthe reading of the phase to determine the tank level. The reading of thephase depends on the electrical length of the system. The phasecorresponds to a level of medical fluid in the tank, and thus medicalfluid level can be determined

Compared to control board 300 of FIG. 7, control board 400 of FIG. 11thus contains less circuitry because control board 400 uses fewervoltages and readings than control board 300. For example, FIG. 7requires power detectors that measure magnitudes of incident andreflected waves that FIG. 11 does not require. Because control board 300of FIG. 7 uses additional voltages and readings, control board 300 maybe used to provide a higher resolution sensor in certain embodiments.

Referring now to FIG. 12, an experimental plot for sensor 20 usingcontrol board 400 of FIG. 11 illustrates there is very little differencein the output of the sensor when the conductivity changes from zero to3.26 mS/cm, to 7.45 mS/cm, to 10.38 mS/cm, to 12.38 mS/cm and to 15.13mS/cm. It is also expected that the conductivity could be increased pastfifteen mS/cm without significantly affecting sensor output using thephase shift methodology. When using control board 400 in system 10,conductivity does not come into play, such that the tuning electronicsdescribed above are not needed.

The curve of FIG. 12 flattens out at fill levels 4 to 6, which is due tothe particular structure of transmission lines or electrodes 24 and 26used for the experiment. If needed, the curve can be corrected bychanging the physical structure of one or both of electrodes 24 and 26.Or, like described above for system 10, it may be sufficient to operatewith the changing or diagonal parts of the curve only, here for example,if only the bottom region (empty) and the top region (full) of tank 40are of interest for the particular application.

As described above, advantages of system 10 include that it requiresrelatively simple packaging and implementation, is non-invasive, istolerant to high temperature (e.g., 100° C.) and humidity (e.g., 100%),is relatively low cost, provides good resolution and repeatability,provides continuous, real-time monitoring, and should require relativelylow maintenance.

When the circuit of FIG. 11 is implemented to determine tank level, thecalibration process correlates each phase reading with a tank level. Inone embodiment, the operating frequency of the system is chosen duringcalibration such that the entire range of possible fluid levels andassociated phase shifts falls into a linear portion of the sinusoidalwave describing the phase of the overall impedance of the tank.

FIGS. 13 and 14 illustrate example frequency sweeps used to select anoperating frequency. The entire frequency range is swept when the tankis empty and the phase shift of the signal in the transmission line isread, as shown in FIG. 13. Then, the entire frequency range is sweptwhen the tank is full of water, e.g., reverse osmosis water, and thephase shift of the signal in the transmission line is read, as shown inFIG. 14. The graphs resulting from the two sweeps are plotted together,as shown in FIG. 15, and the two graphs are analyzed for a frequencywhere the two graphs substantially overlap each other. In the exampleplot of FIG. 15, the two graphs substantially overlap at 1.4 GHz, andthus setting 1.4 GHz as the frequency of the VCO ensures that the phaseof the sensor remains substantially linear for all possible tank levelsranging from empty to full. This ensures that each reading of the phasecorresponds to a unique medical fluid level, enhancing the accuracy andreliability of the system.

Referring now to FIGS. 16 to 19, different medical systems employingsystem 10 are illustrated. FIG. 16 illustrates an online hemodialysissystem 150 employing the level or volume detection system 10. Onlinehemodialysis system 150 pulls water via line 152 from a house or tapsupply of water. Line 152 leads to an online water purification system154, which purifies the water to make the water suitable for a dialysistherapy. Filtered water is delivered via line 156 to a hemodialysismachine 160. Filtered water line 156 enters dialysis machine 160 andfeeds a dialysis preparation unit 162, which includes pumps, valves,lines and chemicals needed to create online dialysate from filteredwater traveling through line 156. One suitable dialysis machine 160having a dialysis generation unit 162 is described in U.S. PublicationNo. 2009/0008331, entitled “Hemodialysis Systems and Methods”, filedFeb. 27, 2008, the entire contents of which are hereby incorporated byreference, for environmental purposes related to system 10 and sensor20, which are otherwise fully and completely described herein.

Dialysis machine 160 also includes a bypass 158, which allows purifiedwater to be delivered to dialysate preparation unit 162. A storagecontainer or tank 40 a of dialysis machine 160 can at different timesstore a level and volume of dialysate emanating from preparation unit162 or store a level and volume of purified water via bypass line 158.Dialysis machine 160 uses the mixed dialysate for therapy purposes.Dialysis machine 160 uses the purified water instead for flushing,priming, rinsing, recirculation and disinfection when therapy is notoccurring dialysate is not needed.

Dialysate or purified water is delivered to a dialysate therapy unit164, which heats and delivers dialysate in a controlled and desiredmanner to a dialyzer 166. In the illustrated embodiment, system 150 usesliquid level and volume detection system 10 having sensor 20 placedadjacent or onto storage vessel 40 a. Cable or ribbon 34 extends fromsensor 20 to a safe processing area within the enclosure of dialysismachine 160 and to a control board 100 for example. Control board 100communicates with a processing board 12 in the embodiment shown in FIG.16.

Referring now to FIG. 17, a batch or semi-batch hemodialysis system 170is illustrated. One example batch or semi-batch dialysis system isdescribed in U.S. Pat. No. 7,749,393, entitled “Batch Filtration SystemFor Preparation Of Sterile Fluid For Renal Replacement Therapy”, filedMay 1, 2009, the entire contents of which are incorporated herein byreference for environmental purposes related to system 10 and sensor 20,which are otherwise fully and completely described herein. Dialysissystem 170, like dialysis system 150, includes a water inlet line 152leading to online water purification unit 154 as described above.Filtered water leaves purification unit 154 via purified water line 156but is delivered instead to tank or container 40 b, which holds a supplyof chemicals 172 needed to convert purified water via line 156 intodialysate suitable for use in a dialysis machine 180. Sensor 20 isconnected via cable or ribbon 34 to control card 100, which in turncommunicates with microprocessor and associated memory 12 as has beendescribed herein.

Dialysis machine 180 pulls mixed dialysate from tank or container 40 bvia dialysate inlet line 174. Dialysate inlet line 174 leads to adialysate heating and volume control delivery unit 182. Dialysatedelivery unit 182 delivers heated dialysate in a controlled manner(pressure and flow) to dialyzer 166 and likewise removes dialysate fromdialyzer 166 in a like controlled manner, for example, removing adesired amount of utrafiltration from the patient.

Sensing system 10 is provided here to detect the level of purified waterfrom unit 154 that has been filled within tank or container 40 b. Sensor20 measures the fluid level rising initially to a point at which it isknown that the concentrate chemicals 172 have been diluted to asufficient level for use for therapy. This level can be confirmed ifneeded via one or more temperature compensated conductivity readings.Level sensor 20 and sensing system 10 are then used during therapy, andperhaps over multiple therapies if enough batch dialysate has been made,to determine how much mixed dialysate remains within container 40 b. Itis contemplated for example that if sensing system 10 indicates that notenough dialysate is left within batch container 40 b for an ongoing ornew therapy, the patient is notified accordingly.

Referring now to FIG. 18, system 10 is illustrated in operation with abagged peritoneal dialysis system 190. One suitable peritoneal dialysissystem 190 is described in Patent Cooperation Treaty (“PCT”) PublicationNo. WO 2009/094183, entitled “Fluid Line Autoconnect Apparatus AndMethods For Medical Treatment System”, filed Jan. 23, 2009 (in PCT), theentire contents which are incorporated herein by reference forenvironmental purposes related to system 10 and sensor 20, which areotherwise fully and completely described herein.

Peritoneal dialysis system 190 includes a plurality of supply bags 192 ato 192 c of premixed dialysate that is suitable for injection into thepatient's peritoneum. Each of supply bags 192 a to 192 c is preconnectedto a dialysate pumping cassette 194 via supply lines 196 a to 196 c,respectively. A peritoneal dialysis cycler 200 is provided to operatepumping and valving cassette 194. In particular, cycler 200 pulls fluidfrom one of supply bags 192 a to 192 c and delivers that fluid viaheater line 198 to a dialysate heating vessel 40 c. When the fluidwithin heating vessel 40 c is heated to a desired level, cycler 200pulls heated dialysate back from heater line 198 into cassette 194 andthen out to the patient.

Sensing system 10 uses sensor 20 as described herein to determine howmuch fluid has been delivered to and removed from dialysate heatingcontainer 40 c. In the illustrated embodiment, sensor 20 communicateswith control board 100 via electrical lines within cable or ribbon 34.Control board 100 in turn communicates via processing unit 12.

It should be appreciated that while premixed dialysate is used in oneembodiment, it is also expressly contemplated to pull (i) dialysateconstituents from different bags 192 a to 192 c or (ii) premixeddialysates having different dextrose or glucose levels from thedifferent bags to produce and mix the constituents or the differentdialysates within heating/mixing container 40 c. Sensing system 10 isused to know how much of each constituent or dialysate has been pumpedfrom any of the bags 192 a to 192 c into container 40 c. For example,premixed dialysates having different dextrose or glucose levels can bepulled in desired amounts from bags 192 a to 192 c to produce a desiredhybrid dialysate within mixing container 40 c, which is then heated fordelivery to the patient. Or, peritoneal dialysis constituents that arenot stable if premixed can be pulled from separate bags.

Referring now to FIG. 19, dialysate sensing system 10 is used with amedical fluid delivery system 210, which can deliver one or more drugsvia drug containers 212, 214, 216 and 218 intravenously to the patientvia a drug infusion pump 240. One suitable embodiment for drug infusionpump 240 is described in U.S. Pat. No. 6,269,340, filed Oct. 27, 1997,entitled “Infusion Pump With An Electronically Loadable Drug Library AndA User Interface For Loading The Library”, the entire contents of whichare incorporated herein by reference for environmental purposes relatedto system 10 and sensor 20, which are otherwise fully and completelydescribed herein. The different drugs are selectively pumped to a drugholding tank 40 d via fluid lines 222, 224, 226 and 228, respectively.The infusion pump selectively pumps from the drug supplies via theopening and closing of valves 232, 234, 236 and 238, respectively. Apump such as a peristaltic pump 244 of infusion pump 240 pumps a drug ormixture thereof from holding tank 40 d to the patient.

Similar to the peritoneal dialysis system 190 of FIG. 18, the drugdelivery system 210 of FIG. 19 can deliver drugs sequentially fromsupplies 212 to 218 through drug holding tank 40 d to the patient.Alternately, drug constituents or premixed drugs are mixed withincontainer 40 d. Sensor 20 monitors the total amount of fluid withinholding tank 40 d. Sensor 20 also meters in precise amounts of drugsfrom any of supplies 212 to 218 in combination to arrive at a desireddrug or drug mixture within container 40 d. Sensing system 10 includessensor 20 connected to control board 100 via cable or ribbon 34. Controlboard 100 in turn communicates with processing unit 12.

Aspects of the subject matter described herein may be useful alone or incombination one or more other aspect described herein. Without limitingthe foregoing description, in a first aspect of the present disclosure,a medical fluid system includes a container holding a fluid at a level;a power source that supplies an input signal at a selected operatingfrequency; a radio frequency level sensor operably connected to thecontainer and including an emitting electrode and a receiving electrode,the electrodes operating as a transmission line; a bi-directionalcoupler operably connected to the sensor, the coupler receiving theinput signal and outputting an incident signal and a reflected signal,the incident signal including a first phase and the reflected signalincluding a second phase; and a phase detector that receives the firstphase and the second phase and outputs a difference signal representinga difference between the first phase and second phase, wherein thedifference signal is associated with the level of the fluid in thecontainer.

In accordance with a second aspect of the present disclosure, which maybe used in combination with any one or more of the preceding aspects,the system is configured and arranged to select the operating frequencyby performing a first frequency sweep of the container when thecontainer is empty to read the phase of the transmission line,performing a second frequency sweep of the container when the containeris full to read the phase of the transmission line, and selecting thefrequency at which the first frequency sweep and second frequency sweepproduce overlapping phase shift results.

In accordance with a third aspect of the present disclosure, which maybe used in combination with any one or more of the preceding aspects,the operating frequency is selected to be about 1.4 GHz, the distancebetween the emitting electrode and receiving electrode is selected to bebetween about 0.08 inches to about 0.31 inches, the emitting electrodeis selected to be about 0.47 inches wide, the receiving electrode isselected to be about 0.47 inches wide, and the emitting electrode andthe receiving electrode are selected to be about 0.08 inches from thecontainer.

In accordance with a fourth aspect of the present disclosure, which maybe used in combination with any one or more of the preceding aspects,the emitting electrode and receiving electrode extend a lengthindicative of a full level of the fluid-holding container.

In accordance with a fifth aspect of the present disclosure, which maybe used in combination with any one or more of the preceding aspects,the system includes a processor operable with the sensor and configuredto operate with stored data associating the difference signal with alevel of the fluid in the container.

In accordance with a sixth aspect of the present disclosure, which maybe used in combination with any one or more of the preceding aspects,the fluid level is a first fluid level and the first fluid level insidethe tank is indicative of a first load of the transmission line.

In accordance with a seventh aspect of the present disclosure, which maybe used in combination with any one or more of the preceding aspects,fluid at a second fluid level inside the tank is indicative of a secondload of the transmission line.

In accordance with an eighth aspect of the present disclosure, which maybe used in combination with any one or more of the preceding aspects,the input signal generates an electric field surrounding the emittingelectrode and the receiving electrode and also generates a radiofrequency wave propagating between the emitting electrode and receivingelectrode in a direction substantially perpendicular to a plane definingthe level of the fluid in the container.

In accordance with a ninth aspect of the present disclosure, which maybe used in combination with any one or more of the preceding aspects,the transmission line is a slot line transmission line.

In accordance with a tenth aspect of the present disclosure, which maybe used in combination with any one or more of the preceding aspects,the impedance seen by the transmission line is indicative of anequivalent dielectric constant ∈, the equivalent dielectric constant ∈based on the dielectric constants ∈₀ and ∈_(d).

In accordance with an eleventh aspect of the present disclosure, whichmay be used in combination with any one or more of the precedingaspects, the equivalent dielectric constant

$ɛ = {\frac{{ɛ\; o} + {ɛ\; d}}{2}.}$

In accordance with a twelfth aspect of the present disclosure, which maybe used in combination with any one or more of the preceding aspects,the value of the dielectric constant ∈_(d) varies based on a level ofthe fluid in the container.

In accordance with a thirteenth aspect of the present disclosure, whichmay be used in combination with any one or more of the precedingaspects, a medical fluid system a container holding a fluid, the fluidat a first time having a first conductivity, the fluid at a second timehaving a second conductivity; and a radio frequency level sensorpositioned in operable relation with the container, the radio frequencyoperation of the level sensor configured to be (i) indicative of a levelor volume of the fluid in the container and (ii) at least substantiallyindependent of whether the fluid has the first conductivity or thesecond conductivity, wherein the radio frequency level sensor includes aradio frequency signal emitting electrode spaced adjacent to a radiofrequency signal receiving electrode and the emitting and receivingelectrodes model a slot line transmission line.

In accordance with a fourteenth aspect of the present disclosure, whichmay be used in combination with any one or more of the preceding aspectsin combination with the thirteenth aspect, the impedance seen by a radiofrequency wave propagating between the emitting and receiving electrodeschanges with the level of the fluid inside the container.

In accordance with a fifteenth aspect of the present disclosure, whichmay be used in combination with any one or more of the preceding aspectsin combination with the thirteenth aspect, the system generates anincident wave and a reflected wave, and a processor is configured toprovide a signal indicative of a phase shift between the incident waveand the reflected wave.

In accordance with a sixteenth aspect of the present disclosure, whichmay be used in combination with any one or more of the preceding aspectsin combination with the thirteenth aspect, the signal indicative of thephase shift corresponds to a level of a fluid in the container.

In accordance with a seventeenth aspect of the present disclosure, whichmay be used in combination with any one or more of the preceding aspectsin combination with the thirteenth aspect, the processor is configuredto provide a signal indicative of a ratio of the magnitudes of theincident wave and the reflected wave.

In accordance with an eighteenth aspect of the present disclosure, whichmay be used in combination with any one or more of the preceding aspectsin combination with the thirteenth aspect, the processor is configuredto provide a signal indicative of an impedance based on the ratio of themagnitudes of the incident wave and the reflected wave and the phaseshift.

In accordance with a nineteenth aspect of the present disclosure, whichmay be used in combination with any one or more of the preceding aspectsin combination with the thirteenth aspect, the signal is indicative ofthe impedance corresponds to a level of a fluid in the container.

In accordance with a twentieth aspect of the present disclosure, any ofthe structure and functionality illustrated and described in connectionwith FIG. 1 may be used in combination with any one or more of thepreceding aspects.

In accordance with a twenty-first aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 2 may be used in combination with any one or moreof the preceding aspects.

In accordance with a twenty-second aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 3 may be used in combination with any one or moreof the preceding aspects.

In accordance with a twenty-third aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 4 may be used in combination with any one or moreof the preceding aspects.

In accordance with a twenty-fourth aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 5 may be used in combination with any one or moreof the preceding aspects.

In accordance with a twenty-fifth aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 6 may be used in combination with any one or moreof the preceding aspects.

In accordance with a twenty-sixth aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 7 may be used in combination with any one or moreof the preceding aspects.

In accordance with a twenty-seventh aspect of the present disclosure,any of the structure and functionality illustrated and described inconnection with FIG. 8 may be used in combination with any one or moreof the preceding aspects.

In accordance with a twenty-eighth aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 9 may be used in combination with any one or moreof the preceding aspects.

In accordance with a twenty-ninth aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 10 may be used in combination with any one or moreof the preceding aspects.

In accordance with a thirtieth aspect of the present disclosure, any ofthe structure and functionality illustrated and described in connectionwith FIG. 11 may be used in combination with any one or more of thepreceding aspects.

In accordance with a thirty-first aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 12 may be used in combination with any one or moreof the preceding aspects.

In accordance with a thirty-second aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 13 may be used in combination with any one or moreof the preceding aspects.

In accordance with a thirty-third aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 14 may be used in combination with any one or moreof the preceding aspects.

In accordance with a thirty-fourth aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 15 may be used in combination with any one or moreof the preceding aspects.

In accordance with a thirty-fifth aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 16 may be used in combination with any one or moreof the preceding aspects.

In accordance with a thirty-sixth aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 17 may be used in combination with any one or moreof the preceding aspects.

In accordance with a thirty-seventh aspect of the present disclosure,any of the structure and functionality illustrated and described inconnection with FIG. 18 may be used in combination with any one or moreof the preceding aspects.

In accordance with a thirty-eighth aspect of the present disclosure, anyof the structure and functionality illustrated and described inconnection with FIG. 19 may be used in combination with any one or moreof the preceding aspects.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

The invention is claimed as follows:
 1. A medical fluid systemcomprising: a container holding a fluid at a level; a power source thatsupplies an input signal at a selected operating frequency; a radiofrequency level sensor operably connected to the container and includingan emitting electrode and a receiving electrode, the electrodesoperating as a transmission line having an electrical impedance thatvaries with the level or volume of the fluid in the container; abi-directional coupler operably connected to the sensor, the couplerreceiving the input signal and outputting an incident signal and areflected signal, the incident signal including a first phase and thereflected signal including a second phase; and a phase detector thatreceives the first phase and the second phase and outputs a differencesignal representing a difference between the first phase and secondphase, wherein the difference signal is associated with the level orvolume of the fluid in the container.
 2. The medical fluid system ofclaim 1, which is configured and arranged to select the operatingfrequency by performing a first frequency sweep of the container whenthe container is empty to read the phase of the transmission line,performing a second frequency sweep of the container when the containeris full to read the phase of the transmission line, and selecting thefrequency at which the first frequency sweep and second frequency sweepproduce overlapping phase shift results.
 3. The medical fluid system ofclaim 2, wherein selecting the frequency at which the first frequencysweep and second frequency sweep produce overlapping phase shift resultsallows the difference signal to correspond to a specific containerlevel.
 4. The medical fluid system of claim 1, wherein the operatingfrequency is selected to be substantially 1.4 GHz, the distance betweenthe emitting electrode and receiving electrode is selected to be betweensubstantially 0.08 inches to substantially 0.31 inches, the emittingelectrode is selected to be substantially 0.47 inches wide, thereceiving electrode is selected to be substantially 0.47 inches wide,and the emitting electrode and the receiving electrode are selected tobe substantially 0.08 inches from the container.
 5. The medical fluidsystem of claim 1, wherein the emitting electrode and receivingelectrode extend a length indicative of a full level of thefluid-holding container.
 6. The medical fluid system of claim 1, whichincludes a processor operable with the sensor and configured to operatewith stored data associating the difference signal with a level of thefluid in the container.
 7. The medical fluid system of claim 1, thefluid level being a first fluid level and wherein the first fluid levelinside the container is indicative of a first load of the transmissionline.
 8. The medical fluid system of claim 7, wherein fluid at a secondfluid level inside the container is indicative of a second load of thetransmission line.
 9. The medical fluid system of claim 1, wherein theinput signal generates an electric field surrounding the emittingelectrode and the receiving electrode and also generates a radiofrequency wave propagating between the emitting electrode and receivingelectrode in a direction substantially perpendicular to a plane definingthe level of the fluid in the container.
 10. The medical fluid system ofclaim 1, wherein the transmission line is a slot line transmission line.11. The medical fluid system of claim 1, wherein the impedance seen bythe transmission line is indicative of an equivalent dielectric constant∈, the equivalent dielectric constant ∈ based on the dielectricconstants ∈_(o) and ∈_(d).
 12. The medical fluid system of claim 11,wherein the equivalent dielectric constant$ɛ = {\frac{{ɛ\; o} + {ɛ\; d}}{2}.}$
 13. The medical fluid system ofclaim 12, wherein the value of the dielectric constant ∈_(d) variesbased on a level of the fluid in the container.
 14. The medical fluidsystem of claim 1, the fluid at a first time having a firstconductivity, the fluid at a second time having a second conductivity;and the radio frequency operation of the level sensor configured to beat least substantially independent of whether the fluid has the firstconductivity or the second conductivity.
 15. The medical fluid system ofclaim 1, wherein the radio frequency level sensor is located outside ofthe container.
 16. The medical fluid system of claim 2, whereinselecting the frequency at which the first frequency sweep and secondfrequency sweep produce overlapping phase shift results allows the firstand second phases to be substantially linear for container levelsranging from empty to full.